Electrical cables for data transmission are well known. One common cable is a coaxial cable. Coaxial cables generally comprise an electrically conductive wire surrounded by an insulator. The wire and insulator are surrounded by a shield, and the wire, insulator and shield are surrounded by a jacket, Coaxial cables are widely used and best known for cable television signal transmission and ethernet standard communications in local area networks. Coaxial cables can transmit at much higher frequencies than a standard twisted pair wire and, therefore, have a much greater transmission capacity. Coaxial cables provide data transmission at raw data rates of up to 10 Mbit/sec (Mbps). In addition, coaxial cables have very little distortion, cross-talk or signal loss, and therefore, provide a very reliable medium for data transmission. Other types of cables are also well known, such as twisted pair cables used for telephone signal transmission, and fiber optic cables.
With the proliferation of high-speed, powerful personal computers and the availability of advanced telecommunications equipment, there is a need for cables that are capable of transmitting data at ever faster speeds. Fiber optic cables provide optimum bandwidth and performance for long distance and high data rate transmissions, since fiber optic cables provide transmission with low attenuation and virtually no noise. Fiber optic cables provide data transmission at data rates up to and beyond 1 Gbit/sec (Gbps). However, despite the increased availability of fiber optic cables, the price of fiber optic cables and particularly transceivers have not dropped to a level where it is always practical to use, especially at short distances. Accordingly, other less expensive cables capable of high speed data transmission are still in demand.
One such cable used for high speed data transmission between two points or devices is a parallel pair or twin axial cable. Parallel pair cable designs provide two separately insulated conductors arranged side by side in parallel relation, the pair being then wrapped in a shield. This style cable is often used in computers, telecommunications and automatic test equipment where high data rate, high fidelity signal transmission is required.
Parallel pair cables are often used for differential signal transmission. In differential signal transmission, two conductors are used for each data signal transmitted and the information conveyed is represented as the difference in voltage between the two conductors. The data is represented by polarity reversals on the wire pair, unlike a coaxial cable where data is represented by the polarity of the center conductor with respect to ground. Thus, the amplitude of the ground potential on a shielded pair cable is not significant as long as it is not so high as to cause electrical breakdown in the receiver circuitry. The receiver only needs to determine whether the relative voltage between the two conductors is that appropriate to a logical 0 or 1. Accordingly, differential signal transmission provides a better signal-to-noise ratio than voltage level to ground signal transmission (also called single-ended transmission) because the signal voltage level is effectively doubled by transmitting the signal simultaneously over both conductors, with one conductor transmitting the signal 180 degrees out of phase from the other. Differential signal transmission provides a balanced signal that is relatively immune to noise and cross-talk. Interfering signals (or "noise") are generally voltages relative to ground and will affect both conductors equally. Since the receiver takes the difference between the two received voltages, the noise components added to the transmitted signal (on each wire) are negated. This noise is called common-mode noise, and the differential property of the receiver which negates the effect of this noise is known as common mode noise rejection. A Standard for differential transmission systems is EIA standard RS-422.
In order to transmit the differential signal along a twin axial cable effectively, the signals on each conductor must propagate down the wire with very low skew. The amount of differential skew per unit length that is allowable is inversely proportional to both the distance of the cable and the data rate at which the signal is transmitted. For example, when transmitting at a data rate of 1000 Mbps, the bit width is approximately 1000 pSec wide. If the difference between the two signals on the differential cable is greater than 200 pSec, errors in communication may occur. If the differential signal is being transmitted 30 meters, then the safe maximum skew would be less than 7 pSec/meter.
Unfortunately, for most existing twin axial cables, typical differential skew is about 16-32 pSec/meter. This type of skew level limits the use-length of 1000 Mbps data transmission to less than 6 meters. As is discussed above, this significantly exceeds the safe level of skew for greater cable lengths. Accordingly, existing twin axial cables are restricted in their ability to effectively transmit differential signals at a high data rate over an extended length.
Low differential skew is also required for proper cancellation of noise. If signals arrive at the receiver at different times, any coupled noise will not be able to cancel, defeating the primary purpose of a twin axial cable. Furthermore, the emitted noise will increase due to reduced cancellation of the high frequency currents on the cable's shield. The present constraints on managing differential skew in conventional twin axial cables severely limits the use of differential signal transmission in more demanding applications. Accordingly, many designers have been forced to switch to far more expensive fiber optic technology for long distance, high data rate transmission.
Therefore, it would be desirable to provide a cable capable of high data rate differential signal transmission at higher speeds and longer distances than achieved by existing differential pair cables. This requires having lower differential skew between paired conductors and lower attenuation than is achieved by existing differential pair cables and providing lower interference from cross-talk and intermodulation noise.
An additional cable construction used for transmitting differential signals is the quad cable. Quad cable designs provide four separately insulated conductors arranged around a central axis at equal circumferential intervals, the insulated conductors then being wrapped in a shield. For moderate data transmission speeds (i.e., less than 200 Mbit/sec), quad cables have been used by transmitting two differential pairs, each pair comprising two conductors, with each conductor oriented generally 180.degree. apart from the other in the pair. The advantage to this type of transmission line is that by having two differential pairs within a single shield, the overall cable size is reduced by approximately 40% when compared with using two separate twin axial cables. This allows for reduced cost and ease of routing cables.
Quad cables today have not been used beyond 200 Mbit/sec data rates because of signal degradation resulting from cross-talk and pulse attenuation. While twin-axial cables typically have equal or lower signal attenuation, when compared with a coax cable of equivalent conductor size, dielectric and shield materials, and impedance, quad cables typically have higher attenuation than a similarly constructed coax. This problem is exaggerated when using relatively inexpensive polyester backed foil shields due to the relatively high resistance in these types of materials. Attenuation will limit both the maximum data rate of transmission as well as the maximum distance of transmission.
Furthermore, differential skew within the quad cable will result in cross-talk between the two differential pairs in the cable. This requires precise control of the balance of material properties and construction within the quad cable in order to achieve adequate performance at longer lengths or higher data rates. Today, the maximum performance specified for a quad cable is 20 meters at 200 Mbit/sec. It would be desirable to provide a cable capable of higher data rate transmission, having the same or smaller size than the quad cable, that is capable of longer distance transmission without significantly increasing the cable cost.